The present invention relates to delivery hoses for the passage therethrough of gases and in particular to delivery hoses associated with medical humidifiers.
It has long been appreciated that, when the upper airway of a patient is by-passed for prolonged periods by, for example, an endotracheal tube, it is important that means for the adequate humidification of the gas inhaled by the patient be provided. Such a humidification means is disclosed in U.K. Pat. No. 1448473. This document refers to a particular problem associated with medical humidifiers, namely the problem of "rain-out". Considerable heat losses can occur during the passage of a humidified gas when it leaves the humidifying chamber and passes along a delivery hose towards the patient. Such heat losses can result in condensation of water taking place inside the delivery hose with subsequent danger to the patient.
In order to overcome the problem of "rain-out", UK Pat. No. 1448473 teaches the use of a linear electrical resistance heater which is provided along the delivery hose and whose heat dissipation is controlled independently of the temperature of the water in the humidifying chamber. Associated with the heater is a temperature sensor situated at the delivery end of hose and a delivery control unit.
There are two problems associated with this known delivery hose. Firstly, the linear electrical resistance wire, together with the sensor renders the hose cumbersome. Secondly, since the sensor is thermally isolated from the heater, it relies on gas flow to indicate the gas temperature. If there is a period in which no gas flows, then the temperature of the gas at the sensor starts to fall and the control unit responds to this fall by increasing the power to the heater. This results in the system heading towards its maximum operating temperature. When the gas flow is turned on again, a transient temperature overshoot occurs.
U.K. Pat. No. 897292 describes an electrically heated flexible hose in which a fabric tape is wound helically around a flexible inner core. Mounted on the tape is a heating wire which is secured to the tape either by being interwoven therewith or by stitching. The heating wire is relatively exposed which necessitates the use of a lagging of heat insulating material which is surrounded by a protective covering. All this renders the hose cumbersome and therefore unsuitable for use in medical applications.